Silicone rubber compositions are known to possess desirable properties in terms of strength, elasticity, and performance over wide ranges of temperature. These properties are generally achieved through the use of finely divided inorganic fillers, e.g., silicas, and chemistries by which the rubbers are rendered infusible and insoluble by crosslinking. Thus, commercially useful silicone rubbers in ultimate applications, including those with high clarity, are both silica-filled and crosslinked.
Recently, a need has been recognized for silicone rubbers which are not permanently crosslinked, but which can be redissolved and reapplied as required in certain coating applications. One approach to such materials is described in U.S. Pat. Nos. 4,493,926, 4,496,705, 4,523,002, and 4,525,567, wherein zwitterionic groups (groups containing both chemically bound positive and negative charges) are used to reversibly crosslink silicone polymers through ionic interactions. Such polymers have pendant ionic groups, i.e., the ionic groups are not part of the polymer backbone, and also do not contain urethane segments.
A variety of silicone-urethane copolymers are known in the art, including some that contain fluorocarbon units, urethane units, urea units, and siloxane units as in U.S. Pat. No. 4,098,742. The latter are disclosed as being useful in providing coatings on a variety of substrates, but have not been examined as bulk silicone rubbers. The same patent describes incorporation of tertiary amine groups in the urethane portion of the backbone. These groups are then quaternized to ammonium groups by acetic acid for the purpose of preparing aqueous dispersions, i.e., such groups provide the property of dispersibility in water.
Similarly, U.S. Pat. No. 4,631,329 discloses silicone-urethane copolymers wherein the molar ratio of diisocyanate plus chain extender to isocyanate reactive species is at least four times that of all other isocyanate-reactive intermediates where at least 50% of the latter isocyanate-reactive intermediates is a polyol, such as tetramethylene glycol. According to this patent, the high molar ratio is employed to obtain copolymers with high hardness and moisture resistance.
U.S. Pat. No. 3,179,622 discloses moisture-curable isocyanato-functional silicones. These polymers are non-linear, both before and after moisture curing, which occurs through formation of permanent crosslinks. Similarly, U.S. Pat. No. 3,246,048 discloses reactions of hydroxyalkyl-functional silicones with diisocyanates and hydroxylated polyethers. The products of such reactions are used in the preparation of silicone-urethane foams.